The present invention relates to a butterfly valve and, more particularly, to a butterfly valve having a function for measuring a flow rate of a fluid flowing through the butterfly valve. The invention is also concerned with a method of measuring a flow rate with a butterfly valve, as well as a method of controlling a flow rate with a butterfly valve.
In general, a butterfly valve has a valve member which is rotatably mounted in a main body and fixed to a valve shaft which is fixed rotatably to the main body. The valve shaft is operated from outside of the main body manually or by means of a pneumatic or electric actuator so as to rotate the valve member, thus changing the valve opening.
The flow rate of a fluid flowing through such a butterfly valve can be controlled by measuring the flow rate by a measuring means and changing the valve opening based on the measured flow rate.
Such a measurement of the flow rate is conducted by, for example, a flowmeter which is independent from the butterfly valve and installed in the vicinity of the butterfly valve. In this case, the flow rate measured by this flowmeter is regarded as the flow rate of the fluid flowing through the butterfly valve.
A typical flowmeter used for this purpose is a differential pressure type flowmeter comprises an orifice provided in the pipe in which the butterfly valve is installed, and a pressure sensor capable of detecting the differential pressure across the orifice, i.e., between the upstream and downstream sides of the orifice. The flow rate is determined from the detected differential pressure .DELTA.P and a capacity coefficient (Cv value) which is a constant value peculiar to the orifice and determined beforehand through, for example, an experiment.
As a modification of such a differential pressure type flowmeter composed of an orifice and a pressure sensor, in order to attain a compact construction of the system, there is a combination system which makes use of the butterfly valve itself as such an orifice. Namely, such a combination system comprises a pressure sensor capable of detecting the differential pressure of the fluid across the butterfly valve and means for detecting the valve opening of the butterfly valve. A capacity coefficient (Cv value) as function of the valve opening peculiar to this butterfly valve is beforehand determined through, for example, an experiment. Thus, the flow rate is determined from the detected differential pressure .DELTA.P and the value of the capacity coefficient Cv corresponding to the detected valve opening. This combination system is proposed in Japanese Patent Unexamined Publication No. 62-270873 of the same applicant.
Another combination system similar to the above-mentioned combination system of a butterfly valve and a pressure sensor is disclosed in Japanese Utility Model Unexamined Publication No. 62-1117, which is designed to measure the flow rate of a fluid flowing through a gate valve used also as such an orifice. Thus, this combination system comprises a pressure sensor capable of detecting the differential pressure across the gate valve, and means for detecting the valve lift of the gate valve. A capacity coefficient (Cv value) as a function of the valve lift peculiar to this gate valve is beforehand determined through, for example, an experiment. Thus, the flow rate is determined by the detected differential pressure .DELTA.P and the value of the capacity coefficient Cv corresponding to the detected valve lift.
Flowmeters of types other than the above mentioned differential pressure type also have been used. For instance, a system has been known in which the valve opening of a butterfly valve disposed in a line of a conductive fluid is controlled in accordance with the flow rate of the fluid measured by an electromagnetic flowmeter disposed independently in the vicinity of the butterfly valve.
In general, it has been regarded that the torque required for rotating the valve member and for fixing the same with a desired valve opening is preferably small, because the smaller torque requires a smaller external driving torque and, hence, a smaller actuator. The smaller torque is preferred also from the view point of maneuverability.
From this point of view, intense study and development have been made for the purpose of reducing the dynamic torque applied to the valve member around the valve shaft by the fluid flowing through the butterfly valve. For instance, the same applicant has proposed, in Japanese Patent Unexamined Publication Nos. 55-142169 and 56-28355, butterfly valves equipped with seat rings capable of reducing the external driving torque.
Known butterfly valves, including those described hereinbefore, function only as a restriction in which the valve member restricts the flow of the fluid.
Thus, in order to enable a butterfly valve to regulate or control the flow rate, it is necessary to install suitable means for measuring the flow rate in the path of the flow of the fluid in addition to the valve member. A butterfly valve system including such means for measuring the flow rate therefore complicates the construction of the flow passage, failing to meet the demand for a compact design.
It is also to be pointed out that quantitative and time deviations tend to exist between the flow rate measured by the flowmeter and the actual flow rate through the butterfly valve, due to the fact that the flow rate is measured indirectly or at a position which is spaced from the butterfly valve. This makes it difficult to measure and, hence, to control the flow rate accurately.